Piston type pumps are well known in the art and typically include a piston mounted in a cylinder for reciprocal movement within the cylinder. The piston is typically mounted on a piston rod which extends through one end of the cylinder. A seal or packing is disposed around the piston rod where it passes through the end of the cylinder to prevent leakage. An inlet is disposed at one end of the cylinder and an outlet is disposed at the opposite end. As the piston moves in one direction, fluid is drawn into the inlet end of the cylinder and is expelled out the outlet by the piston at the opposite end. When the piston reverses direction, the fluid in the inlet end passes through a check valve in the piston into the outlet end of the chamber. Because the piston rod consumes space in the outlet end, a portion of the fluid is expelled through the outlet. The remaining portion is expelled during the next intake-stroke.
During operation, there is a relatively high pressure on the seal surrounding the piston rod. Eventually, the high pressure will result in failure of the seal and the fluid being pumped leaks out around the piston rod.
One solution which has been tried is to provide a double packing at the end through which the rod passes to form a closed chamber to trap any liquid leaking around the piston rod. A drain passage leads from the closed chamber back to the fluid passage. One such system is disclosed in U.S. Pat. No. 2,147,352. A similar arrangement is also shown in U.S. Pat. No. 3,293,994 which incorporates a "trap chamber" in a double-acting hydraulic cylinder for trapping any fluid which leaks around the piston rod.